The present invention relates to a method of developing an electrostatic latent image on the surface of an image-bearing member with a magnetic developer attracted on the surface of a developer-transporting roll made of an integrally-molded cylindrical permanent magnet.
In an electrophotographic or electrostatic imaging process, an electrostatic latent image on a photoconductive or dielectric surface of an image-bearing member is developed by bringing a magnetic brush of a magnetic developer into contact with the latent image while employing a developing means comprising a permanent magnet member concentrically mounted within a sleeve which is rotatable relatively with the permanent magnet member. Then, the developed toner image is fixed directly or after transferred onto a recording sheet such as plain paper to give a final image.
However, the magnetic brush system likely causes background fogging because the magnetic brush is brought into contact with not only the latent image portion but also non-image portion. To avoid this problem, an electric field generated by a current of D.C. bias superimposed with A.C. bias is applied to the region between the image-bearing member and the sleeve.
FIG. 2 is a schematic cross sectional view showing an electrophotographic imaging apparatus to practice the conventional method. In FIG. 2, a magnetic developer 2 is stored in a toner storage 1. The lower portion of the toner storage 1 partially receives a developing roll 6 comprising a cylindrical permanent magnet member 4 recessed with a plurality of permanent magnets 3 and a hollow cylindrical sleeve 5 made of a non-magnetic metal material such as SUS304. The permanent magnet member 4 is concentrically mounted within the sleeve 5 which is rotatable relative to the permanent magnet member 4.
A photosensitive drum 7 is rotatable in the direction indicated by an arrow and opposed to the developing roll 6 through a gap g. The thickness of a magnetic developer layer on the sleeve 5 is regulated by a doctor blade 8 positioned at an end of the toner storage 1 and opposed to the developing roll 6 through a gap t. An alternating current supply 9 and a direct current supply 10 are connected between the photoconductive drum 7 and the doctor blade 8 to apply a D.C./A.C. superimposed bias. Generally, the gap g is slightly larger than the gap t.
As the sleeve 5 rotates in the direction indicated by an arrow while keeping the permanent magnet member 4 stationary, the magnetic developer 2 is attracted onto the sleeve 5 and transported to a developing zone opposite to the photoconductive drum 7. In the developing zone, a toner in the magnetic developer 2 is attracted to the latent image portion on the photoconductive drum 7 by the force received from the electric field generated by the latent image overcoming the attractive force from the permanent magnet member 4 to the surface of the sleeve 5. The latent image is developed in this manner.
In the above conventional method, the magnetic developer 2 is attracted on the surface of the sleeve 5 by the attractive force from the permanent magnet member 4 and transported by a frictional force between the sleeve surface and the attracted magnetic developer. Therefore, the exterior circumferential surface of the sleeve 5 is roughened by a blast finishing to effectively transport the magnetic developer 2. However, since the coefficient of friction becomes low due to wearing of the sleeve surface with the use, the thickness of the magnetic developer layer changes to result in deteriorated developability.
In addition, the developing roll 6 is assembled by mounting the permanent magnet member 4 concentrically within the sleeve 5. This complicated assembly operation leads to increased production cost.
In order to miniaturize a printer, etc., proposed is a sleeveless method in which only the rotatable permanent magnet member 4 is employed to develop the latent image by a magnetic brush system (for example, JP-A-62-201463). In this method, the upper half of the magnetic brush is brought into contact with the surface of the photoconductive drum 7.
However, the above sleeve-less magnetic brush method involves a problem of uneven image density, and particularly in forming halftone image, an image of deteriorated quality is produced because of the differences in the heights and developability between the magnetic brushes positioned on the magnetic poles and those positioned between the magnetic poles. The uneven image density may be avoided by rotating the permanent magnet member 4 at higher speed. However, this makes the driving torque larger and generates a tremendous noise.
Therefore, the peripheral speed Vm of the permanent magnet member 4 is usually set to at least about 1.5 times the peripheral speed Vp of the photoconductive drum 7. However, since the magnetic developer 2 on the permanent magnet member 4 is always brought into contact with the surface of the photoconductive drum 7, the magnetic developer 2 is swept toward the rotation direction of the permanent magnet member 4 to result in deteriorated image quality. Further, the increased peripheral speed Vm involves another problem of uneven image density, namely, a printed image, especially in a solid black image, has higher density in a portion developed later as compared with the other portion in the printed image.
To remove this defect, it has been proposed to set the moving speed of the magnetic developer nearly the same as or less than 1.9 times the peripheral speed of the photoconductive drum 7 (JP-B-63-39910 and JP-A-6-274025). However, the apparatus used therein has a developing roll comprising a permanent magnet member and a sleeve, and therefore, involves the problem of complicated assembly operation. In particular, since the apparatus disclosed in JP-B-63-39910 is operated by rotating both the permanent magnet member and the sleeve, a more complicated driving system is required, thus preventing the miniaturization of the apparatus.
In addition to the above method, also proposed is a sleeveless magnetic brush method in which only a rotatable magnetic roller, at least the exterior circumferential portion thereof being made of an electrically-conductive rubber containing uniformly dispersed magnetic powder, is employed (for example, JP-A-53-42738).
JP-A-53-42738 discloses that uneven image density occurs when the moving speed of magnetic toner on the rotating magnetic roller is the same as the moving speed of the photoconductive surface, because the toner layer has uneven thickness in the developing zone, this causing a variable amount of the magnetic toner adhering to the photoconductive surface. To eliminate this defect, JP-A-53-42738 discloses that the photoconductive surface and the magnetic toner are preferred to move in the same direction, and the moving speed V.sub.T of the magnetic toner is preferably larger than the moving speed Vo of the photoconductive surface. Also, it is taught that the best developing effect can be achieved when the moving speeds satisfy the relation, 5.gtoreq.V.sub.T /Vo.gtoreq.1.5.
However, it is necessary to increase the number of rotations of the magnetic roller in order to satisfy the relation, V.sub.T =1.5.times.Vo to 5.times.Vo. The increasing in the number of rotation brings about increases in the driving torque and generation of tremendous noise. Another problem in this method is a defective printed image such as white streak caused by the adhesion of the toner to the doctor blade or the variation in the toner flowability.